1. Field of the Invention
This invention relates to ceramic capacitors. More particularly, this invention relates to a ceramic capacitor possessing in situ high voltage surge protection (varistor characteristic) made using thin films of ZnO-based ceramic dielectric material.
2. Description of the Related Art
Capacitors may be damaged when subjected to an excessive voltage stress, i.e., a voltage surge above the rated voltage of the device. The effects on the capacitor of such excessive voltage surges can be eliminated by using an external non-linear resistive device across the capacitor to shunt the high voltage.
Such a non-linear resistive device, known as a varistor, exhibits high resistance to low voltages. However, when the voltage exceeds a predetermined threshold, the resistance of the varistor rapidly changes permitting a high voltage to easily pass through the device.
Such devices are described in Carlson et al U.S. Pat. Nos. 4,452,728 and 4,452,729 as well as in Gupta et al U.S. Pat. No. 4,460,497. These patents describe the construction of non-linear resistive devices or varistors using a metal oxide mixture of at least about 90 mole % ZnO with the balance comprising other metal oxides including aluminum oxide and a minor amount of either boron oxide or an alkali metal oxide or both.
While such external devices are capable of protecting a capacitor from excessive voltage surges, both economics and the ever increasing demand for miniaturization of electronic components make it desirable to produce a capacitor possessing in situ high voltage surge protection properties, i.e., a self-limiting capacitor.
A zinc oxide-based varistor, like any metal oxide ceramic material having metal electrodes attached thereto, exhibits some capacitance due to the dielectric properties of the zinc oxide, particularly across the grain boundaries. However, the capacitance of such a varistor, which is conventionally formed as a disc device with a thickness of as much as 0.4 to 4 cm to provide the desired low voltage resistance, is normally about tens of picofarads per cm.sup.2, which is too low to be of value in normal applications for a self-limiting capacitor.
It is known that monolithic multilayer ceramic capacitors can be made by using layers of ceramic films coated with electrodes. These are then stacked together to form a capacitor. The electrodes on the layers are alternately stacked to contact the outer edge of opposite sides of the stack to form a group of parallel capacitors. This construction, while permitting the use of thin films to increase the capacitance, is preferred over a single film of increased area due to the fragility of the thin ceramic film.
It would, however, be desirable to provide a selflimiting capacitive device having in situ high voltage surge protection while providing a useful level of capacitance.